In reading Hippolytus we come across the expression "this day" or the like repeatedly –– line 22 (ἐν τῆιδ’ ἡμέραι), 55 (φάος … τόδε), 355 (ἐχθρὸν ἦμαρ, ἐχθρὸν εἰσορῶ φάος), 369 (παναμέριος ὅδε χρόνος), 726 (τῆιδ’ ἐν ἡμέραι) and 889-90 (ἡμέραν … τήνδ᾽). Indeed such expressions are stock ones in Greek literature, but in Hippolytus they all appear in very important places where the story is progressing to the next stage, which suggests that they work in some effective way in the course of the play. The aim of this paper is to point out this fact and make clear how they work. The first two expressions in question (22, 55) are found in the opening speech by Aphrodite, where the goddess declares that she will punish Hippolytus –– this is the main theme of the play –– and that, in order to realize her plan, she has made Phaedra –– who is a virtuous woman –– fall in love with him. So Phaedra is dying with her passion concealed from anyone around her, but the goddess predicts that her passion will be disclosed to Theseus soon. The audience, at the beginning of the play, know that this play will be developed from concealment of Phaedra’s passion to its disclosure –– through the power of Aphrodite. After the goddess' speech, the play develops in the direction of disclosure as the goddess says. The nurse, who is trying to know what her mistress is worrying about, finally finds out that Phaedra is in love with Hippolytus. Now the story of the play is turning to a new stage where Phaedra’s passion has become known to other people. At this turning point, we encounter the third and fourth instance of the expression (355, 369). The fifth one (726) is seen at the next turning point, where Phaedra's passion has been revealed to Hippolytus, and the sixth (889-90) is at a further turning point, where she has died with the δέλτος –– through which she tells her husband a falsehood that Hippolytus raped or was trying to rape her. In such ways, "this day" expressions in Hippolytus are found at the turning points of the play. Then, how do the expressions work? We should notice that they are all put with references to the power of Aphrodite –– which is the cause of the action, as is shown in the opening speech. That means that the audience is reminded of the cause of the action every time the story is turning to the next stage. Why should they be reminded? I think that is because the character of Phaedra in this play is totally different from the older portrayal of her –– she was a notorious "bad woman" while in this play she is virtuous –– and the constant confirmation of this fact is a part of Euripides’ strategy for advertising the novelty of his portrayal.